The present invention generally relates to the field of wireless communication systems. It particularly refers to a spatial diversity transmitter ad a spatial diversity receiver in a wireless communication system, as well as a corresponding method for wirelessly transmitting ad receiving modulated RF signals. The present invention is specifically advantageous but not exclusively limited to the application in a multiple-input multiple-output (MIMO) spatial multiplexing system and enables reception and transmission of said RF signals via multiple wireless signal propagation paths of a multipath fading channel in a way that correlation between the MIMO channel components are reduced and/or the signal to interference plus noise ratio (SINR) is increased. This results in an improved bit error rate (BER) or packet error rate (PER) performance of said wireless MIMO spatial multiplexing system.
Spatial diversity techniques have been shown to provide an effective means to combat multipath fading and to significantly mitigate co-channel interference in a wireless communication system. Nowadays, multiple-input multiple-output (MIMO) spatial multiplexing systems with a first antenna array consisting of N≧2 transmit (Tx) antennas on a wireless transmitter side and a second antenna array consisting of M≧2 receive (Rx) antennas on a wireless receiver side are becoming increasingly popular, which is owing to the fact that these systems provide an increased date rate R without the need for an increase in signal bandwidth B since the applied transmit antennas are able to transmit independent data streams. These independent data streams, however, interfere with each other such that a complex channel equalizer is required at the wireless receiver side in order to separate RF signals received via different signal propagation paths. Furthermore, to obtain optimum performance it is important that the different signal propagation paths between the transmit antennas of the antenna array on wireless transmitter side and receiver array on the wireless receiver array are uncorrelated with respect to each other. For handheld phone and portable terminals where only compact space diversity receivers can be used (and antenna elements are typically spaced by a distance of a fraction of the RF signal wavelength), the propagation paths become correlated and performance is degraded.
In conventional MIMO spatial multiplexing systems according to the prior art in which spatial antenna diversity is applied by using multiple-element antenna arrays with N omni-directional transmit antennas on a wireless transmitter side (each transmit antenna transmitting a different data symbol) and M omni-directional receive antennas on a wireless receiver side a maximum data transmission rate R can be achieved which is N times greater than the data transmission rate in an equivalent non-MIMO wireless communication system.
The system behavior of a MIMO spatial multiplexing system can be described by the matrix-vector equationR(ƒ,t0)=H(ƒ,t0)·S(ƒ)+N(ƒ)[√{square root over (W)}·Hz−1]  (1a)with R(ƒ,t0):=[R1(ƒ,t0), . . . , Rm(ƒ,t0), RM(ƒ,t0)]TεCM being an M-dimensional vector representing the obtained RF signals {Rm(ƒ,t0)}mε{1, . . . , M} at the input ports of a spatial diversity receiver's M demodulator stages at t=t0, S(ƒ):=[S1(ƒ), . . . , Sn(ƒ), . . . , SN(ƒ)]TεCN being an N-dimensional vector representing the signal spectra {Sn(ƒ)}nε{1, . . . , N} of the particular RF signals {sn(t)}nε{1, . . . , N} to be transmitted by N antenna elements of a spatial diversity transmitter's antenna array (on the assumption that Sn(ƒ)≡S(ƒ)∀n ε{1, 2, 3, . . . , N}, wherein S(ƒ) denotes the corresponding signal spectrum of an information-carrying modulated RF signal s(t) to be transmitted) and N(ƒ):=[N1(ƒ), . . . , Nm(ƒ), . . . , NM(ƒ)]TεCM being an M-dimensional vector representing additive white Gaussian noise components {Nm(ƒ)}mε{1, . . . , M} at t=t0. In this equation, the multipath propagation channel transfer function at observation time t=t0 is given in a matrix form
                                                        H                              _                _                                      ⁡                          (                              f                ,                                  t                  0                                            )                                :=                                    (                                                                                                                  H                        11                                            ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                                                                  ⋯                                                                                                      H                                                  1                          ⁢                          n                                                                    ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                                                                  ⋯                                                                                                      H                                                  1                          ⁢                          N                                                                    ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                                                                                                  ⋮                                                        ⋱                                                        ⋮                                                        ⋯                                                        ⋮                                                                                                                                      H                                                  m                          ⁢                                                                                                          ⁢                          1                                                                    ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                                                                  ⋯                                                                                                      H                                                  m                          ⁢                                                                                                          ⁢                          n                                                                    ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                                                                  ⋯                                                                                                      H                                                  m                          ⁢                                                                                                          ⁢                          N                                                                    ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                                                                                                  ⋮                                                        ⋯                                                        ⋮                                                        ⋱                                                        ⋮                                                                                                                                      H                                                  M                          ⁢                                                                                                          ⁢                          1                                                                    ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                                                                  ⋯                                                                                                      H                                                  M                          ⁢                                                                                                          ⁢                          n                                                                    ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                                                                  ⋯                                                                                                      H                                                  M                          ⁢                                                                                                          ⁢                          N                                                                    ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                                                                        )                        ∈                          C                              M                ×                N                                                    ⁢                                  ⁢        with                            (                  1          ⁢          b                )                                                                                                      H                  mn                                ⁡                                  (                                      f                    ,                                          t                      0                                                        )                                            =                                                Re                  ⁢                                      {                                                                  H                        mn                                            ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                              }                                                  +                                                      j                    ·                    Im                                    ⁢                                      {                                                                  H                        mn                                            ⁡                                              (                                                  f                          ,                                                      t                            0                                                                          )                                                              }                                                                                                                                          =                                                                                                                                                    H                          mn                                                ⁢                                                  (                                                      f                            ,                                                          t                              0                                                                                )                                                                                                            ·                                          ⅇ                                                                        j                          ·                          ∠                                                ⁢                                                                                                  ⁢                                                                              H                            mn                                                    ⁡                                                      (                                                          f                              ,                                                              t                                0                                                                                      )                                                                                                                                ⁢                                      ∀                                          m                      ∈                                              {                                                  1                          ,                          …                          ⁢                                                                                                          ,                          M                                                }                                                                                                        ,                              n                ∈                                  {                                      1                    ,                    …                    ⁢                                                                                  ,                    N                                    }                                            ,                                                          (                  1          ⁢          c                )            wherein {Hmn(ƒ,t0)}mε{1, . . . , M}, nε{1, . . . , N} denote the L=M·N complex-valued channel transfer functions for the L strongest signal propagation paths between the N transmit antenna elements of said spatial diversity transmitter and the M receive antenna elements of said spatial diversity receiver in the above-described wireless MIMO spatial multiplexing system at t=t0, |Hmn(ƒ, t0)| and ∠Hmn(ƒ, t0) denote the corresponding frequency response and phase response of channel transfer function Hmn(ƒ, t0) at t=t0, respectively, and j:=√{square root over (−1)} is the imaginary unit.